Method and apparatus for antenna diversity in multi-input multi-output communication systems

ABSTRACT

Transmission schemes that can flexibly achieve the desired spatial multiplexing order, spatial diversity order, and channel estimation overhead order are described. For data transmission, the assigned subcarriers and spatial multiplexing order (M) for a receiver are determined, where M≧1. For each assigned subcarrier, M virtual antennas are selected from among V virtual antennas formed with V columns of an orthonormal matrix, where V≧M. V may be selected to achieve the desired spatial diversity order and channel estimation overhead order. Output symbols are mapped to the M virtual antennas selected for each assigned subcarrier by applying the orthonormal matrix. Pilot symbols are also mapped to the V virtual antennas. The mapped symbols are provided for transmission from T transmit antennas, where T≧V. Transmission symbols are generated for the mapped symbols, e.g., based on OFDM or SC-FDMA. Different cyclic delays may be applied for the T transmit antennas to improve diversity.

The present application claims priority to U.S. patent application Ser.No. 11/261,823, entitled “Method and Apparatus for Providing AntennaDiversity in a Wireless Communication System,” filed Oct. 27, 2005 whichclaims priority to provisional U.S. Application Ser. No. 60/710,408,entitled “Method and Apparatus for Antenna Diversity in Multi-inputMulti-Output Communication Systems,” filed Aug. 22, 2005, andprovisional U.S. Application Ser. No. 60/711,144 entitled “Method andApparatus for Antenna Diversity in Multi-input Multi-OutputCommunication Systems,” filed Aug. 24, 2005, both assigned to theassignee hereof and incorporated herein by reference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to transmission schemes for wireless communication.

II. Background

In a wireless communication system, a transmitter (e.g., a base stationor a terminal) may utilize multiple (T) transmit antennas for datatransmission to a receiver equipped with one or more (R) receiveantennas. The multiple transmit antennas may be used to increase systemthroughput by transmitting different data from these antennas and/or toimprove reliability by transmitting data redundantly. For example, thetransmitter may transmit a given symbol from all T transmit antennas,and the receiver may receive multiple versions of this symbol via the Rreceive antennas. These multiple versions of the transmitted symbolgenerally improve the receiver's ability to recover the symbol.

Transmission performance may be improved by exploiting the spatialdimension obtained with the multiple transmit antennas and, if present,the multiple receive antennas. A propagation path exists between eachpair of transmit and receive antennas. T·R different propagation pathsare formed between the T transmit antennas and the R receive antennas.These propagation paths may experience different channel conditions(e.g., different fading, multipath, and interference effects) and mayachieve different signal-to-noise-and-interference ratios (SNRs). Thechannel responses for the T·R propagation paths may vary from path topath and may further vary across frequency for a dispersive wirelesschannel and/or over time for a time-variant wireless channel.

A major drawback to using multiple transmit antennas for datatransmission is that the channel response between each pair of transmitand receive antennas (or each propagation path) typically needs to beestimated in order to properly receive the data transmission. Estimationof the full channel response for all T·R transmit and receive antennapairs may be undesirable for several reasons. First, a large amount oflink resources may be consumed in order to transmit a pilot used forchannel estimation, which in turn reduces the link resources availableto transmit data. Second, channel estimation for all T·R transmit andreceive antenna pairs increases processing overhead at the receiver.

There is therefore a need in the art for transmission schemes that canameliorate the need to estimate the full channel response for alltransmit and receive antenna pairs.

SUMMARY

Transmission schemes that can flexibly achieve the desired spatialmultiplexing order, spatial diversity order, and channel estimationoverhead order are described herein. The spatial multiplexing orderdetermines the number of symbols to send simultaneously on onesubcarrier in one symbol period, the spatial diversity order determinesthe amount of spatial diversity observed by the transmitted symbols, andthe channel estimation overhead order determines the amount of pilotoverhead.

In an embodiment, for a data transmission from a transmitter to areceiver, the subcarriers assigned to the receiver and the spatialmultiplexing order (M) for the receiver are determined, where M≧1. Foreach assigned subcarrier, M virtual antennas are selected from among Vvirtual antennas formed with V columns of an orthonormal matrix, whereV≧M. V may be selected to achieve the desired spatial diversity orderand channel estimation overhead order. The M virtual antennas for eachassigned subcarrier may be selected in various manners, as describedbelow. Output symbols for the receiver are mapped to the M virtualantennas selected for each assigned subcarrier by applying theorthonormal matrix. Pilot symbols are also mapped to the V virtualantennas. The mapped output symbols and pilot symbols (or transmitsymbols) are provided for transmission from T physical transmitantennas, where T≧V. Transmission symbols (e.g., OFDM symbols or SC-FDMAsymbols) are generated for each transmit antenna based on the transmitsymbols for that transmit antenna. Different cyclic delays may beapplied to the transmission symbols for the T transmit antennas.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout.

FIG. 1 shows a wireless communication system.

FIGS. 2A and 2B show MISO and MIMO channels, respectively.

FIG. 3 shows a transmission scheme with virtual antennas.

FIG. 4 shows a transmission scheme with virtual antennas and cyclicdelay diversity.

FIG. 5 shows a MIMO transmission by cycling through the virtualantennas.

FIGS. 6A, 6B and 6C show three exemplary subcarrier structures.

FIG. 7 shows an exemplary frequency hopping scheme.

FIG. 8 shows an exemplary pilot scheme for symbol rate hopping.

FIG. 9A through 9D show four exemplary pilot schemes for block hopping.

FIG. 10 shows a process for transmitting data and pilot to one or morereceivers.

FIG. 11 shows an apparatus for transmitting data and pilot to one ormore receivers.

FIG. 12 shows a block diagram of a base station and two terminals.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a wireless communication system 100 with multiple basestations 110 and multiple terminals 120. A base station is a stationthat communicates with the terminals. A base station may also be called,and may contain some or all of the functionality of, an access point, aNode B, and/or some other network entity. Each base station 110 providescommunication coverage for a particular geographic area 102. The term“cell” can refer to a base station and/or its coverage area depending onthe context in which the term is used. To improve system capacity, abase station coverage area may be partitioned into multiple smallerareas, e.g., three smaller areas 104 a, 104 b, and 104 c. Each smallerarea is served by a respective base transceiver subsystem (BTS). Theterm “sector” can refer to a BTS and/or its coverage area depending onthe context in which the term is used. For a sectorized cell, the BTSsfor, all sectors of that cell are typically co-located within the basestation for the cell. The transmission techniques described herein maybe used for a system with sectorized cells as well as a system withun-sectorized cells. For example, the techniques may be used for thesystem described in the aforementioned U.S. patent application Ser. No.______ [Attorney Docket No. 05091]. For simplicity, in the followingdescription, the term “base station” is used generically for a BTS thatserves a sector as well as a base station that serves a cell.

Terminals 120 are typically dispersed throughout the system, and eachterminal may be fixed or mobile. A terminal may also be called, and maycontain some or all of the functionality of, a mobile station, a userequipment, and/or some other device. A terminal may be a wirelessdevice, a cellular phone, a personal digital assistant (PDA), a wirelessmodem card, and so on. Each terminal may communicate with zero, one, ormultiple base stations on the downlink and uplink at any given moment.The downlink (or forward link) refers to the communication link from thebase stations to the terminals, and the uplink (or reverse link) refersto the communication link from the terminals to the base stations.

For a centralized architecture, a system controller 130 couples to basestations 110 and provides coordination and control for these basestations. For a distributed architecture, the base stations maycommunicate with one another as needed.

The transmission techniques described herein may be used for variouswireless communication systems such as an orthogonal frequency divisionmultiple access (OFDMA) system, a single-carrier frequency divisionmultiple access (SC-FDMA) system, a frequency division multiple access(FDMA) system, a code division multiple access (CDMA) system, a timedivision multiple access (TDMA) system, a spatial division multipleaccess (SDMA) system, and so on. An OFDMA system utilizes orthogonalfrequency division multiplexing (OFDM), which is a multi-carriermodulation technique that partitions the overall system bandwidth intomultiple (K) orthogonal subcarriers. These subcarriers may also becalled tones, bins, and so on. With OFDM, each subcarrier is associatedwith a respective subcarrier that may be modulated with data. An SC-FDMAsystem may utilize interleaved FDMA (IFDMA) to transmit on subcarriersthat are distributed across the system bandwidth, localized FDMA (LFDMA)to transmit on a block of adjacent subcarriers, or enhanced FDMA (EFDMA)to transmit on multiple blocks of adjacent subcarriers. In general,modulation symbols are sent in the frequency domain with OFDM and in thetime domain with SC-FDMA.

An OFDM symbol may be generated for one transmit antenna in one symbolperiod as follows. N modulation symbols are mapped to N subcarriers usedfor transmission (or N assigned subcarriers) and zero symbols withsignal value of zero are mapped to the remaining K−N subcarriers. AK-point inverse fast Fourier transform (IFFT) or inverse discreteFourier transform (IDFT) is performed on the K modulation symbols andzero symbols to obtain a sequence of K time-domain samples. The last Qsamples of the sequence are copied to the start of the sequence to forman OFDM symbol that contains K+Q samples. The Q copied samples are oftencalled a cyclic prefix or a guard interval, and Q is the cyclic prefixlength. The cyclic prefix is used to combat intersymbol interference(ISI) caused by frequency selective fading, which is a frequencyresponse that varies across the system bandwidth.

An SC-FDMA symbol may be generated for one transmit antenna in onesymbol period as follows. N modulation symbols to be sent on N assignedsubcarriers are, transformed to the frequency domain with an N-pointfast Fourier transform (FFT) or discrete Fourier transform (DFT) toobtain N frequency-domain symbols. These N frequency-domain symbols aremapped to the N assigned subcarriers, and zero symbols are mapped to theremaining K−N subcarriers. A K-point IFFT or IDFT is then performed onthe K frequency-domain symbols and zero symbols to obtain a sequence ofK time-domain samples. The last Q samples of the sequence are copied tothe start of the sequence to form an SC-FDMA symbol that contains K+Qsamples.

A transmission symbol may be an OFDM symbol or an SC-FDMA symbol. TheK+Q samples of a transmission symbol are transmitted in K+Q sample/chipperiods. A symbol period is the duration of one transmission symbol andis equal to K+Q sample/chip periods.

The transmission techniques described herein may be used for thedownlink as well as the uplink. For clarity, much of the followingdescription is for downlink transmission from a base station (atransmitter) to one or more terminals (receivers). For each subcarrier,the base station may transmit to one terminal without SDMA or tomultiple terminals with SDMA.

FIG. 2A shows a multiple-input single-output (MISO) channel formed bymultiple (T) transmit antennas 112 a through 112 t at base station 110and a single receive antenna 122 x at a terminal 120 x. The MISO channelmay be characterized by a 1×T channel response row vector h(k) for eachsubcarrier k, which may be given as:

h (k)=[h ₁(k)h ₂(k) . . . h _(T)(k)]  Eq (1)

where h_(i)(k), for i=1, . . . , T, denotes the coupling or complexchannel gain between transmit antenna i and the single receive antennafor subcarrier k.

FIG. 2B shows a multiple-input multiple-output (MIMO) channel formed bythe T transmit antennas 112 a through 112 t at base station 110 andmultiple (R) receive antennas 122 a through 122 r at a terminal 120 y.The MIMO channel may be characterized by an R×T channel response matrixH(k) for each subcarrier k, which may be given as:

$\begin{matrix}{{{\underset{\_}{H}(k)} = {\begin{bmatrix}{h_{1,1}(k)} & {h_{1,2}(k)} & \ldots & {h_{1,T}(k)} \\{h_{2,1}(k)} & {h_{2,2}(k)} & \ldots & {h_{2,T}(k)} \\\vdots & \vdots & \ddots & \vdots \\{h_{R,1}(k)} & {h_{R,2}(k)} & \ldots & {h_{R,T}(k)}\end{bmatrix} = \lbrack {{{\underset{\_}{h}}_{1}(k)}\mspace{14mu} {{\underset{\_}{h}}_{2}(k)}\mspace{14mu} \ldots \mspace{14mu} {{\underset{\_}{h}}_{T}(k)}} \rbrack}},} & {{Eq}\mspace{14mu} (2)}\end{matrix}$

-   where h_(j,i)(k), for j=1, . . . , R and i=1, . . . , T, denotes the    complex channel gain between transmit antenna i and receive antenna    j for subcarrier k; and    -   h _(i)(k) is an R×1 channel response vector for transmit antenna        i, which is the i-th column of H(k).

The transmitter may transmit one or more output symbols from the Ttransmit antennas on each subcarrier in each symbol period. Each outputsymbol may be a modulation symbol for OFDM, a frequency-domain symbolfor SC-FDMA, or some other complex value. The data transmission may bequantified by the following metrics:

-   -   Spatial multiplexing order (M)—the number of output symbols        transmitted via the T transmit antennas on one-subcarrier in one        symbol period;    -   Spatial diversity order (D)—the amount of spatial diversity        observed by the transmitted output symbols; and    -   Channel estimation overhead order (C)—the number of virtual        antennas to be estimated by a receiver for each receive antenna.        In general, M≦min {T, R}, D≦T, and C≦T. The spatial diversity        refers to transmit diversity resulting from the use of multiple        transmit antennas and does not include receive diversity        resulting from the use of multiple receive antennas.

If the transmitter transmits output symbols directly from the T transmitantennas, then a receiver typically needs to estimate the full channelresponse for all T transmit antennas in order to recover the datatransmission. The channel estimation overhead order is then C=T. Incertain scenarios, it may be desirable to transmit fewer than T outputsymbols simultaneously, e.g., if the channel conditions are poor. Asubset of the T transmit antennas may be used to transmit fewer than Toutput symbols. However, this is undesirable since the transmit powersavailable for the unused transmit antennas are not judiciously employedfor transmission.

The transmission schemes described herein allow for flexible selectionof the three metrics M, D and C in order to achieve good performance fordata transmission in different conditions. For example, a larger spatialmultiplexing order M may be selected for good channel conditions withhigh SNRs, and a smaller spatial multiplexing order may be selected forpoor channel conditions with low SNRs. A lower channel estimationoverhead Order C may be selected, e.g., in scenarios where lowthroughput due to low SNRs does not justify a large channel estimationoverhead.

The transmission schemes described herein can utilize all T transmitantennas for transmission, regardless of the number of output symbolsbeing sent and regardless of which subcarriers are used fortransmission. This capability allows the transmitter to utilize all ofthe transmit power available for the T transmit antennas, e.g. byutilizing the power amplifiers coupled to each of the antennas, fortransmission, which generally improves performance. Employing fewer thanT transmit antennas for transmission typically results in less than allof the available transmit power being used for the transmission, whichwould impact performance.

The transmission schemes described herein can readily support MIMO,single-input multiple-output (SIMO), and single-input single-output(SISO) transmissions. A MIMO transmission is a transmission of multipleoutput symbols from multiple virtual antennas to multiple receiveantennas on one subcarrier in one symbol period. A SIMO transmission isa transmission of a single output symbol from one virtual antenna tomultiple receive antennas on one subcarrier in one symbol period. A SISOtransmission is transmission of a single output symbol from one virtualantenna to one receive antenna on one subcarrier in one symbol period.The transmitter may also send a combination of MIMO, SIMO and/or SISOtransmissions to one or more receivers in one symbol period.

The transmitter may transmit M output symbols simultaneously from the Ttransmit antennas on one subcarrier in one symbol period using varioustransmission schemes. In an embodiment, the transmitter processes theoutput symbols for transmission, as follows:

x (k)= U·P (k)· s (k),  Eq (3)

where

-   -   s(k) is an M×1 vector containing M output symbols to be sent on        subcarrier k in one symbol period;    -   P(k) is a V×M permutation matrix for subcarrier k;    -   U=[u ₁ u ₂ . . . u _(V)] is a T×V orthonormal matrix; and    -   x(k) is a T×1 vector containing T transmit symbols to be sent        from the T transmit antennas on subcarrier k in one symbol        period.        V is the number of virtual antennas formed with the orthonormal        matrix U. In general, 1≦M≦V≦T. V may be a fixed value or a        configurable value.

The orthonormal matrix U is characterized by the property U ^(H)·U=I,where “^(H)” denotes a conjugate transpose and I is the identity matrix.The V columns of U are orthogonal to one another, and each column hasunit power. In an embodiment, U is defined such that the sum of thesquared magnitude of the V entries in each row is equal to a constantvalue. This property results in equal transmit power being used for allT transmit antennas. U may also be a unitary matrix that ischaracterized by the property U ^(H)·U=U·U ^(H)=I. Orthonormal andunitary matrices may be formed as described below. The V columns of Uare used to form V virtual antennas that may be used to send up to Voutput symbols on one subcarrier in one symbol period. The virtualantennas may also be called effective antennas or by some otherterminology.

In an embodiment, a single orthonormal matrix U is used for all K totalsubcarriers in all symbol periods, so that U is not a function ofsubcarrier index k or symbol index n. In another embodiment, differentorthonormal matrices are used for different subcarrier sets that may beassigned to different receivers. In yet another embodiment, differentorthonormal matrices are used for different subcarriers. In yet anotherembodiment, different orthonormal matrices are used for different timeintervals, where each time interval may span one or multiple symbolperiods. In yet another embodiment, one or more orthonormal matrices areselected for use from among multiple orthonormal matrices, as describedbelow. In general, data and pilot may be transmitted using one or moreorthonormal matrices such that a receiver is able to estimate, thechannel response based on the pilot and use the channel responseestimate to recover the data sent to the receiver.

The permutation matrix P(k) selects which M virtual antennas to use forsubcarrier k from among the V virtual antennas available for use, orwhich M of the V columns of U. The permutation matrix P(k) may bedefined in various manners, and different permutation matrices may beused for different subcarriers, as described below.

FIG. 3 shows a model 300 for the transmission scheme given by equation(3). The transmitter receives the data vector s(k) for each subcarrierand symbol period used for transmission. A virtual antenna mapper 310processes the data vector s(k) and generates the transmit vector x(k).Within virtual antenna mapper 310, a symbol-to-virtual antenna mappingunit 312 multiplies the data vector s(k) with the permutation matrixP(k) and generates a V×1 intermediate vector. A spatial spreading unit314 multiplies the intermediate vector with the orthonormal matrix U andgenerates the transmit vector x(k). The transmit vector x(k) istransmitted from the T transmit antennas and via a MIMO channel 350 to Rreceive antennas at a receiver.

The received symbols at the receiver may be expressed as:

$\begin{matrix}\begin{matrix}{{{\underset{\_}{r}(k)} = {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{x}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{\underset{\_}{H}(k)} \cdot \underset{\_}{U} \cdot {\underset{\_}{P}(k)} \cdot {\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{{\underset{\_}{H}}_{eff}(k)} \cdot {\underset{\_}{P}(k)} \cdot {\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{{\underset{\_}{H}}_{used}(k)} \cdot {\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},}\end{matrix} & {{Eq}\mspace{14mu} (4)}\end{matrix}$

where

-   -   r(k) is an R×1 vector containing R received symbols from the R        receive antennas on subcarrier kin one symbol period;    -   H _(eff)(k) is an R×V effective channel response matrix for        subcarrier k,    -   H _(used)(k) is an R×M used channel response matrix for        subcarrier k, and    -   n(k) is an R×1 noise vector for subcarrier k.

The effective and used channel response matrices may be given as:

$\begin{matrix}{\begin{matrix}{{{{\underset{\_}{H}}_{eff}(k)} = {{\underset{\_}{H}(k)} \cdot \underset{\_}{U}}},} \\{{= \lbrack {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{u}}_{1}}\mspace{14mu} {{\underset{\_}{H}(k)} \cdot {\underset{\_}{u}}_{2}}\mspace{14mu} \ldots \mspace{14mu} {{\underset{\_}{H}(k)} \cdot {\underset{\_}{u}}_{v}}} \rbrack},}\end{matrix}{and}} & {{Eq}\mspace{14mu} (5)} \\\begin{matrix}{{{{\underset{\_}{H}}_{used}(k)} = {{{\underset{\_}{H}}_{eff}(k)} \cdot {\underset{\_}{P}(k)}}},} \\{{= \lbrack {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{u}}_{(1)}}\mspace{14mu} {{\underset{\_}{H}(k)} \cdot {\underset{\_}{u}}_{(2)}}\mspace{14mu} \ldots \mspace{14mu} {{\underset{\_}{H}(k)} \cdot {\underset{\_}{u}}_{(M)}}} \rbrack},}\end{matrix} & {{Eq}\mspace{14mu} (6)}\end{matrix}$

where {u ₍₁₎ u ₍₂₎ . . . u _((M)}⊂{u) ₁ u ₂ . . . u _(V)}.

As shown in equation (3) and illustrated in FIG. 3, an effective MIMOchannel with V virtual antennas is formed by the use of the orthonormalmatrix U. Data is sent on all or a subset of the V virtual antennas. Aused MIMO channel is formed by the M virtual antennas used fortransmission.

For the transmission scheme described above, an R×T MIMO system iseffectively reduced to an R×V MIMO system. The transmitter appears as ifit has V virtual antennas rather than T transmit antennas, where V≦T.This transmission scheme decreases the channel estimation overhead orderto C=V. However, the spatial multiplexing order is limited to V, or M≦V,and the spatial diversity order is also limited to V, or D≦V.

The description above is for one subcarrier k. The transmitter mayperform the same processing for each subcarrier used for transmission.The frequency diversity of each virtual antenna across subcarriers isthe same as the frequency diversity of the physical transmit antennas.However, the spatial diversity is reduced from T to V.

In another embodiment, the transmitter processes the output symbols fortransmission, as follows:

{tilde over (x)} (k)= D (k)· U·P (k)· s (k),  Eq (7)

where D(k) is a T×T diagonal matrix for sub carrier k. D(k) is used toachieve cyclic delay diversity, which improves the frequency selectivityof the virtual antennas and may improve spatial diversity order tosomewhere between V and T. Cyclic delay diversity may be achieved in thetime domain or the frequency domain.

Cyclic delay diversity may be achieved in the time domain by circularlyshifting (or cyclically delaying) the sequence of K time-domain samples(obtained from the K-point IDFT or IFFT) for each transmit antenna i bya delay of T_(i), for i=1, . . . T. For example, T_(i) may be defined asT_(i)=(i−1)·J, where J may be equal to one sample period, a fraction ofa sample period, or more than one sample period. J may be selected suchthat the channel impulse response for each virtual antenna is expectedto be shorter than the cyclic prefix length. A cyclic delay of X samplesmay be achieved by moving the last X samples in the sequence of Ktime-domain samples to the front of the sequence. The time-domainsamples for the T transmits antenna are cyclically delayed by differentamounts. A cyclic prefix may be appended after applying the cyclic delayin order to ensure orthogonality among the K total subcarriers.

Cyclic delay diversity may also be achieved in the frequency domain byapplying a phase ramp (or a progressive phase shift) across the K totalsubcarriers for each transmit antenna. T different phase ramps are usedfor the T transmit antennas to achieve K different cyclic delays forthese antennas. The diagonal matrix D(k) for each subcarrier k may bedefined as follows:

$\begin{matrix}{{{\underset{\_}{D}(k)} = \begin{bmatrix}1 & 0 & \ldots & 0 \\0 & ^{{j2\pi} \cdot {({k - 1})} \cdot {J/T}} & \ldots & 0 \\\vdots & \vdots & \ddots & \vdots \\0 & 0 & \ldots & ^{{{j2\pi} \cdot {({k - 1})} \cdot {({T - 1})}}{J/T}}\end{bmatrix}},{{{for}\mspace{14mu} k} = 1},\ldots \mspace{14mu},{K.}} & {{Eq}\mspace{14mu} (8)}\end{matrix}$

As indicated by equation (8), transmit antenna 1 has a phase slope of 0across the K total subcarriers, transmit antenna 2 has a phase slope of2π·J/T across the K total subcarriers, and so on, and transmit antenna Thas a phase slope of 2π·(T−1)·J/T across the K total subcarriers. Thediagonal matrix D(k) and the orthonormal matrix U may also be combinedto obtain a new orthonormal matrix U(k)=D(k)·U, where U(k) may beapplied to the data vector s(k).

The received symbols with cyclic delay diversity may be expressed as:

$\begin{matrix}\begin{matrix}{{{\underset{\_}{\overset{\sim}{r}}(k)} = {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{\overset{\sim}{x}}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot \underset{\_}{U} \cdot {\underset{\_}{P}(k)} \cdot {\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{{\underset{\_}{\overset{\sim}{H}}}_{eff}(k)} \cdot {\underset{\_}{P}(k)} \cdot {\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},} \\{{= {{{{\underset{\_}{\overset{\sim}{H}}}_{used}(k)} \cdot {\underset{\_}{s}(k)}} + {\underset{\_}{n}(k)}}},}\end{matrix} & {{Eq}\mspace{14mu} (9)}\end{matrix}$

where

-   -   {tilde over (r)}(k) is an R×1 received vector with cyclic delay        diversity;    -   {tilde over (H)} _(eff)(k) is an R×V effective channel response        matrix with cyclic delay diversity; and    -   {tilde over (H)} _(used)(k) is an R×M used channel response        matrix with cyclic delay diversity.

The effective and used channel response matrices may be given as:

$\begin{matrix}{\begin{matrix}{{{{\underset{\_}{\overset{\sim}{H}}}_{eff}(k)} = {{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot \underset{\_}{U}}},} \\{= \lbrack {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot {\underset{\_}{u}}_{1}}\mspace{14mu} {{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot {\underset{\_}{u}}_{2}}\mspace{14mu} \ldots} } \\{ {{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot {\underset{\_}{u}}_{v}} \rbrack,}\end{matrix}{and}} & {{Eq}\mspace{14mu} (10)} \\\begin{matrix}{{{{\underset{\_}{\overset{\sim}{H}}}_{used}(k)} = {{{\underset{\_}{\overset{\sim}{H}}}_{eff}(k)} \cdot {\underset{\_}{P}(k)}}},} \\{= \lbrack {{{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot {\underset{\_}{u}}_{(1)}}\mspace{14mu} {{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot {\underset{\_}{u}}_{(2)}}\mspace{14mu} \ldots}\mspace{14mu} } \\{ {{\underset{\_}{H}(k)} \cdot {\underset{\_}{D}(k)} \cdot {\underset{\_}{u}}_{(M)}} \rbrack.}\end{matrix} & {{Eq}\mspace{14mu} (11)}\end{matrix}$

FIG. 4 shows a model 400 for the transmission scheme given by equation(7). Within a virtual antenna mapper 410, a symbol-to-virtual antennamapping unit 412 multiplies the data vector s(k) with the permutationmatrix P(k) and generates a V×1 vector. A spatial spreading unit 414multiplies the V×1 vector with the orthonormal matrix U and generates aT×1 vector. A cyclic delay diversity unit 416 multiplies the T×1 vectorwith the diagonal matrix D(k) and generates the T×1 transmit vectorx(k). The transmit vector x(k) is transmitted from the T transmitantennas and via a MIMO channel 450 to R receive antennas at a receiver.

As shown in equation (7) and illustrated in FIG. 4, an effective MIMOchannel {tilde over (H)} _(eff)(k) with V virtual antennas is formed bythe use of the orthonormal matrix U and cyclic delay diversity. A usedMIMO channel {tilde over (H)} _(used)(k) is formed by the M virtualantennas used for transmission.

Equations (3) and (7) assume that equal transmit power is used for the Moutput symbols being sent simultaneously on one subcarrier in one symbolperiod. In general, the transmit power available for each transmitantenna may be uniformly or non-uniformly distributed across thesubcarriers used for transmission. The transmit powers available for theT transmit antennas for each subcarrier may be uniformly ornon-uniformly distributed to the M output symbols being sent on thatsubcarrier. Different transmit powers may be used for the M outputsymbols by scaling the data vector s(k) with a diagonal gain matrix G asfollows: x(k)=U·P(k)·G·s(k) or {tilde over (x)}(k)=D(k)·U·P(k)·G·s(k),where diag{G}={g₁ g₂ . . . g_(M)} and g₁ is the gain for output symbols_(i).

Various types of matrices may be used to form the orthonormal matrix U.For example, U may be formed based on a Fourier matrix, a Walsh matrix,or some other matrix. A T×T Fourier matrix F _(T×T) has element f_(n,m)in the n-th row of the m-th column, which may be expressed as:

$\begin{matrix}{{f_{n,m} = ^{{- {j2\pi}}\frac{{({n - 1})}{({m - 1})}}{T}}},{{{for}\mspace{14mu} n} = 1},\ldots \mspace{14mu},{{T\mspace{14mu} {and}\mspace{14mu} m} = 1},\ldots \mspace{14mu},{T.}} & {{Eq}\mspace{14mu} (12)}\end{matrix}$

Fourier matrices of any square dimension (e.g., 2, 3, 4, 5, 6, and soon) may be formed. A 2×2 Walsh matrix W _(2×2) and larger size Walshmatrix W _(2N×2N) may be expressed as:

$\begin{matrix}{{{\underset{\_}{W}}_{2 \times 2} = {\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\mspace{14mu} {and}}}\mspace{14mu} {{\underset{\_}{W}}_{2\; N \times 2\; N} = {\begin{bmatrix}{\underset{\_}{W}}_{N \times N} & {\underset{\_}{W}}_{N \times N} \\{\underset{\_}{W}}_{N \times N} & {- {\underset{\_}{W}}_{N \times N}}\end{bmatrix}.}}} & {{Eq}\mspace{14mu} (13)}\end{matrix}$

In an embodiment, the orthonormal matrix U is equal to a matrixcontaining V columns of a T×T Fourier matrix or a T×T Walsh matrix. Inanother embodiment, U is formed as follows:

U=Λ·F,  Eq (14)

where F is a T×V matrix containing the first V columns of the T×TFourier matrix; and

Λ is a T×T diagonal matrix containing T scaling values for the T rows ofF.

For example, the diagonal matrix Λ may be defined as Λ=diag{1 e^(jθ) ¹ .. . e^(jθ) ^(T) }, where θ_(i) for i=1, . . . , T may be random phases.Equation (14) multiplies the rows of F with random phases, which changesthe spatial directions depicted by the columns of F. In yet anotherembodiment, U is an orthonormal matrix with pseudo-random elements,e.g., having unit magnitude and pseudo-random phases.

The transmitter may send a MIMO, SIMO or SISO transmission to a receiveron a set of subcarriers, which are called the assigned subcarriers. TheK total subcarriers may be partitioned into multiple non-overlappingsubcarrier sets. In this case, the transmitter may transmit to multiplereceivers simultaneously on multiple subcarrier sets. The transmittermay send the same or different types of transmission to these multiplereceivers. For example, the transmitter may send a MIMO transmission ona first subcarrier set to a first receiver, a SIMO transmission on asecond subcarrier set to a second receiver, a SISO transmission on athird subcarrier set to a third receiver, and so on.

A SIMO or SISO transmission may be sent from a single virtual antennaformed with a single column of the orthonormal matrix U. In this case,M=V=1, and the effective MIMO channel becomes an R×1 SISO or SIMOchannel having a channel response vector of h _(eff)(k)=H(k)·u ₁ or{tilde over (h)}(k)=H(k)·D(k)·u ₁. The data vector s(k) becomes a 1×1vector containing a single output symbol, the permutation matrix P(k)becomes a 1×1 matrix containing a single ‘1’, and the orthonormal matrixU becomes a T×1 matrix containing a single column.

A MIMO transmission may be sent from multiple virtual antennas formedwith multiple columns of the orthonormal matrix U. If the number ofoutput symbols is less than the number of virtual antennas (or M<S),then M virtual antennas may be selected for use in various manners.

FIG. 5 shows an embodiment for transmitting output symbols cyclicallyfrom the V virtual antennas. For this embodiment, the first M outputsymbols are sent from virtual antennas 1 through M on the first assignedsubcarrier, the next M output symbols are sent from virtual antennas 2through M+1 on the next assigned subcarrier, and so on. The assignedsubcarriers may be given indices of k=1, 2, . . . . For the embodimentshown in FIG. 5, the M virtual antennas used for subcarrier k+1 areoffset by one from the M virtual antennas used for subcarrier k. Theselected virtual antennas wrap around to virtual antenna 1 upon reachingthe last virtual antenna. Hence, virtual antennas ((k−1)mod V)+1 through((k+M−2)mod V)+1 are used for assigned subcarrier k, where “mod S”denotes a modulo-S operation and the “−1” and “+1” are due to the indexfor the assigned subcarriers and the index for the virtual antennasstarting with 1 instead of 0. The M columns of the permutation matrixP(k) for each assigned subcarrier k are the ((k−1, k, k+1, . . . ,k+M−2)mod V)+1 columns of a V×V identify matrix. For example, if M=2 andV=3, then the permutation matrices may be defined as:

$\begin{matrix}{{{\underset{\_}{P}(1)} = \begin{bmatrix}1 & 0 \\0 & 1 \\0 & 0\end{bmatrix}},{{\underset{\_}{P}(2)} = \begin{bmatrix}0 & 0 \\1 & 0 \\0 & 1\end{bmatrix}},{{\underset{\_}{P}(3)} = \begin{bmatrix}0 & 1 \\0 & 0 \\1 & 0\end{bmatrix}},{{\underset{\_}{P}(4)} = \begin{bmatrix}1 & 0 \\0 & 1 \\0 & 0\end{bmatrix}},{{and}\mspace{14mu} {so}\mspace{14mu} {{on}.}}} & {{Eq}\mspace{14mu} (15)}\end{matrix}$

In another embodiment, the first M output symbols are sent from virtualantennas 1 through M on the first assigned subcarrier, the next M outputsymbols are sent from virtual antennas M+1 through ((2M−1)mod V)+1 onthe next assigned subcarrier, and so on. For this embodiment, the Mvirtual antennas used for subcarrier k+1 start after the last virtualantenna used for subcarrier k. In yet another embodiment, the M virtualantennas for each subcarrier are selected in a pseudo-random manner,e.g., based on a pseudo-random number (PN) generator or sequence that isalso known to the receiver.

In yet another embodiment, the virtual antennas are selected based onfeedback from a receiver. For example, the feedback may indicate thespecific virtual antennas to use for all assigned subcarriers, thespecific virtual antennas to use for each assigned subcarrier, and soon. In yet another embodiment, the transmitter may select the virtualantennas based on a pilot or some other transmission received from thereceiver. For example, the transmitter may estimate the uplink channelresponse based on the received pilot, estimate the downlink channelresponse based on the uplink channel response estimate, and select thevirtual antennas based on the downlink channel response estimate. Thedownlink and uplink channel responses may be similar, e.g., in a timedivision duplexed (TDD) system in which downlink and uplinktransmissions are sent on the same frequency channel but in differenttime intervals.

In general, the virtual antennas may be selected (1) by the transmitterin a deterministic manner (e.g., cyclically) or a pseudo-random mannerwithout feedback from the receiver, (2) by the transmitter based onfeedback from receiver, or (3) by the receiver and sent to thetransmitter.

The orthonormal matrix U may be fixed, and the V virtual antennas formedwith U may be selected for use as described above. In anotherembodiment, one or more orthonormal matrices are selected for use fromamong a set of orthonormal matrices available for use. The set oforthonormal matrices forms a codebook, and one or more entries of thecodebook may be used for transmission. The orthonormal matrices in theset are different (and may be pseudo-random) with respect to each other.For example, the orthonormal matrices may be defined to provide goodperformance for different channel conditions, e.g., low and high SNRconditions, low and high mobility, and so on. One orthonormal matrix maybe selected for all assigned subcarriers, for each assigned subcarrier,and so on. The matrix selection may be made (1) by the transmitter withor without feedback from a receiver or (2) by the receiver and sent backto the transmitter. The matrix selection may be made based on variousfactors such as, e.g., the channel conditions, mobility, uplinkresources, and so on. In general, the particular entry or entries in thecodebook to use for transmission may be selected either autonomously bythe transmitter or based on feedback from the receiver.

The transmission schemes described herein has the following desirablefeatures:

-   -   Flexibility to easily select the number of virtual antennas;    -   Flexibility to send any number of output symbols up to the        number of available virtual antennas; and    -   Utilization of all T transmit antennas for transmission        regardless of the number of output symbols being sent and the        number of available virtual antennas.

The number of virtual antennas (V) may be selected to support thedesired spatial multiplexing order (M), to achieve the desired spatialdiversity order (D), and to obtain the desired channel estimationoverhead order (C). The number of virtual antennas may be selectedautonomously by the transmitter or based on a feedback from thereceiver. The desired number of virtual antennas may readily be obtainedby defining the orthonormal matrix U with the proper number of columns.

The spatial multiplexing order is limited by the number of transmitantennas and the number of receive antennas, or M≦min {T, R}. A higherspatial multiplexing order may be desirable in certain scenarios (e.g.,high SNR conditions) and if supported by the receiver. A lower spatialmultiplexing order (e.g., M=1) may be desirable in other scenarios(e.g., low SNR conditions) or if a higher spatial multiplexing order isnot supported by the receiver. The spatial multiplexing order may bedynamically selected based on the channel conditions and/or otherfactors. For example, the spatial multiplexing order may be set to oneif the SNR is less than a first threshold, set to two if the SNR isbetween the first threshold and a second threshold, set to three if theSNR is between the second threshold and a third threshold, and so on.The number of virtual antennas is selected to be equal to or greaterthan the spatial multiplexing order, or V≧M.

In general, a higher spatial diversity order is desirable in order toimprove performance, and a lower channel estimation overhead order isdesirable in order to reduce the amount of link resources used totransmit a pilot for channel estimation. The channel estimation overheadorder is closely related to the spatial diversity order, and both aredetermined by the number of virtual antennas. Hence, the number ofvirtual antennas may be dynamically selected based on the desiredspatial diversity order, the desired channel estimation overhead order,the channel conditions, and/or other factors.

The number of virtual antennas may be selected in various manners. In anembodiment, the number of virtual antennas is set equal to the spatialmultiplexing order, or V=M. In another embodiment, the number of virtualantennas is set to a largest possible value such that the link resourcesused for pilot transmission is maintained within a predeterminedpercentage of the total link resources. In yet another embodiment, thenumber of virtual antennas is set based on the channel conditions. Forexample, one virtual antenna may be defined if the SNR is less than afirst value, two virtual antennas may be defined if the SNR is betweenthe first value and a second value, and so on.

The transmission schemes described herein may be, used with varioussubcarrier structures, some of which are described below. The followingdescription assumes that the K total subcarriers are usable fortransmission and are given indices of 1 through K.

FIG. 6A shows an interlace subcarrier structure 600. For this subcarrierstructure, the K total subcarriers are arranged into S non-overlappinginterlaces, each interlace contains N subcarriers that are uniformlydistributed across the K total subcarriers, and consecutive subcarriersin each interlace are spaced apart by S subcarriers, where K=S·N.Interlace u contains subcarrier u as the first subcarrier, where uε{1, .. . , S}.

FIG. 6B shows a block subcarrier structure 610. For this subcarrierstructure, the K total subcarriers are arranged into S non-overlappingblocks, with each block containing N adjacent subcarriers, where K=S·N .Block ν contains subcarriers ν·N+1 through (ν+1)·N, where νε{1, . . . ,S}.

FIG. 6C shows a group subcarrier structure 620. For this subcarrierstructure, the K total subcarriers are arranged into S non-overlappinggroups, each group contains G subgroups that are distributed across thesystem bandwidth, and each subgroup contains L adjacent subcarriers,where K=S·N and N=G·L. The K total subcarriers may be partitioned into Gfrequency ranges, with each frequency range containing S·L consecutivesubcarriers. Each frequency range is further partitioned into Ssubgroups, with each subgroup containing L consecutive subcarriers. Foreach frequency range, the first L subcarriers are, allocated to group 1,the next L subcarriers are allocated to group 2, and so on, and the lastL subcarriers are allocated to group S. Each group contains G subgroupsof L consecutive subcarriers, or a total of N=G·L subcarriers.

In general, the transmission techniques described herein may be used forany subcarrier structure with any number of subcarrier sets. Eachsubcarrier set may include any number of subcarriers that may bearranged in any manner. For example, a subcarrier set may be equal to aninterlace, a subcarrier block, a subcarrier group, and so on. For eachsubcarrier set, (1) the subcarriers in the set may be uniformly ornon-uniformly distributed across the system bandwidth, (2) thesubcarriers in the set may be adjacent to one another in one group, or(3) the subcarriers in the set may be distributed in multiple groups,where each group may be located anywhere within the system bandwidth andmay contain one or multiple subcarriers.

For all of the subcarrier structures described above, differentreceivers may be assigned different subcarrier sets, and the transmittermay transmit data to each receiver on its assigned subcarrier set. Thetransmitter may use the same orthonormal matrix U for all receivers, adifferent orthonormal matrix for each receiver, a different orthonormalmatrix for each subcarrier set, a different orthonormal matrix for eachsubcarrier, and so on.

The transmission techniques described herein may be used with or withoutfrequency hopping. With frequency hopping, the data transmission hopsfrom subcarrier to subcarrier in a pseudo-random or deterministic mannerover time, which allows the data transmission to better withstanddeleterious channel conditions such as narrowband interference, jamming,fading, and so on. Frequency hopping can provide frequency diversity andinterference randomization. A receiver may be assigned a traffic channelthat is associated with a hop pattern that indicates which subcarrierset(s), if any, to use in each time slot. A hop pattern is also called afrequency hopping pattern or sequence. A time slot is the amount of timespent on a given subcarrier set and is also called a hop period. The hoppattern may select different subcarrier sets in different time slots ina pseudo-random or deterministic manner.

FIG. 7 shows an exemplary frequency hopping scheme 700. In FIG. 7,traffic channel 1 is mapped to a specific sequence of time-frequencyblocks. Each time-frequency block is a specific subcarrier set in aspecific time slot. In the example shown in FIG. 7, traffic channel 1 ismapped to subcarrier set 1 in time slot 1, subcarrier set 4 in time slot2, and so on. Traffic channels 2 through S may be mapped to verticallyand circularly shifted versions of the time-frequency block sequence fortraffic channel 1. For example, traffic channel 2 may be mapped tosubcarrier set 2 in time slot 1, subcarrier set 5 in time slot 2, and soon.

Frequency hopping may be used with any of the subcarrier structuresshown in FIGS. 6A through 6C. For example, a symbol rate hopping schememay be defined in which each time-frequency block is a specificinterlace in one symbol period. For this hopping scheme, the assignedsubcarriers span across the entire system bandwidth and change fromsymbol period to symbol period. As another example, a block hoppingscheme may be defined in which each time-frequency block is a specificsubcarrier block in a time slot of multiple symbol periods. For thishopping scheme, the assigned subcarriers are contiguous and fixed for anentire time slot but changes from time slot to time slot. For the blockhopping scheme, the spatial multiplexing order may be set equal to thenumber of virtual antennas, so that constant interference may beobserved on any given time-frequency block in any sector for a systemwith synchronous sectors. Other hopping scheme may also be defined.

Pilot may be transmitted in various manners with the subcarrierstructures described above. Some exemplary pilot schemes for symbol ratehopping and block hopping are described below.

FIG. 8 shows an exemplary pilot scheme 800 for symbol rate hopping. Forpilot scheme 800, the transmitter transmits a common pilot on oneinterlace from virtual antenna 1 in each symbol period. The transmittermay transmit the common pilot on different interlaces in differentsymbol periods, as shown in FIG. 8. Such a staggered pilot allows areceiver to sample the frequency spectrum on more subcarriers and toderive a longer channel impulse response estimate. The transmitter mayalso transmit an auxiliary pilot on one or more interlaces from theremaining virtual antennas to allow MIMO receivers to estimate thechannel response for all virtual antennas used for transmission. For theembodiment shown in FIG. 8, the transmitter transmits the auxiliarypilot on one interlace in each symbol period and cycles through virtualantennas 2 through V in V−1 different symbol periods. For the case withV=4 as shown in FIG. 8, the transmitter transmits the auxiliary pilotfrom virtual antenna 2 in symbol period n+1, then from virtual antenna 3in symbol period n+2, then from virtual antenna 4 in symbol period n+3,then from virtual antenna 2 in symbol period n+4, and so on.

The transmitter may transmit the common and auxiliary pilots in othermanners. In another embodiment, the auxiliary pilot is staggered andsent on different sets of subcarriers. In yet another embodiment, thecommon pilot is sent on one or more subcarrier sets that arepseudo-random (or have random offsets) with respect to the one or moresubcarrier sets used for the auxiliary pilot.

The transmitter may transmit the common pilot for MIMO, SIMO and SISOreceivers and may transmit the auxiliary pilot only when MIMO receiversare present. The MIMO, SIMO and SISO receivers may use the common pilotto derive a channel estimate for the K total subcarriers of virtualantenna 1. A MIMO receiver may use the auxiliary pilot to derive channelestimates for virtual antennas 2 through V.

FIG. 9A shows an exemplary pilot scheme 910 for block hopping. For theembodiment shown in FIG. 9A, a time-frequency block is composed of 16adjacent subcarriers k+1 through k+16 and further spans 8 symbol periodsn+1 through n+8. For pilot scheme 910, the transmitter transmits adedicated pilot on subcarriers k+3, k+9 and k+15 in each of symbolperiods n+1 through n+3 and n+6 through n+8, or six strips of threepilot symbols. Each pilot symbol may be sent from any virtual antenna.For example, if V=3, then the transmitter may transmit the pilot fromvirtual antenna 1 in symbol periods n+1 and n+6, from virtual antenna 2in symbol periods n+2 and n+7, and from virtual antenna 3 in symbolperiods n+3 and n+8.

FIG. 9B shows an exemplary pilot scheme 920 for block hopping. For pilotscheme 920, the transmitter transmits a dedicated pilot on subcarriersk+3, k+9 and k+15 in each of symbol periods n+1 through n+8, or threestrips of eight pilot symbols. Each pilot symbol may be sent from anyvirtual antenna. For example, if. V=4, then the transmitter may transmitthe pilot from virtual antenna 1 in symbol periods n+1 and n+5, fromvirtual antenna 2 in symbol periods n+2 and n+6, from virtual antenna 3in symbol periods n+3 and n+7, and from virtual antenna 4 in symbolperiods n+4 and n+8.

FIG. 9C shows an exemplary pilot scheme 930 for block hopping. For pilotscheme 930, the transmitter transmits a dedicated pilot on subcarriersk+1, k+4, k+7, k+10, k+13 and k+16 in each of symbol periods n+1, n+2,n+7 and n+8. Each pilot symbol may be sent from any virtual antenna. Forexample, the transmitter may transmit the pilot from virtual antenna 1in symbol period n+1, from virtual antenna 2 in symbol period n+2, fromvirtual antenna 1 or 3 in symbol period n+7, and from virtual antenna 2or 4 in symbol period n+8.

FIG. 9D shows an, exemplary pilot scheme 940 for block hopping. Forpilot scheme 940, the transmitter transmits a staggered pilot on threesubcarriers in each symbol period and on different pilot subcarriers indifferent symbol periods. Each pilot symbol may be sent from any virtualantenna. For example, the transmitter may transmit the pilot from adifferent virtual antenna in each symbol period and may cycle throughthe V virtual antennas in V symbol periods.

In general, for the block hopping scheme, the transmitter may transmit apilot in each time-frequency block such that a receiver, is able toderive a channel estimate for each virtual antenna used fortransmission. FIGS. 9A through 9D show four exemplary pilot patternsthat may be used. Other pilot patterns may also be defined and used forpilot transmission.

For both symbol rate hopping and block hopping, the transmitter maytransmit the pilot from any number of virtual antennas, may use anynumber of pilot subcarriers for each virtual antenna, and may use anyamount of transmit power for each virtual antenna. If the pilot is sentfrom multiple virtual antennas, then the transmitter may use the same ordifferent numbers of subcarriers for these virtual antennas and maytransmit the pilot at the same or different power levels for the virtualantennas. The transmitter may or may not stagger, the pilot for eachvirtual antenna. The transmitter may transmit the pilot on moresubcarriers to allow a receiver to obtain more “look” of the wirelesschannel in the frequency domain and to derive a longer channel impulseresponse estimate. The transmitter may transmit the pilot on all pilotsubcarriers from one virtual antenna in each symbol period, as describedabove. Alternatively, the transmitter may transmit the pilot frommultiple virtual antennas on multiple subsets of subcarriers in a givensymbol period.

In an embodiment, the transmitter transmits the pilot from the virtualantennas, as described above for FIGS. 8 through 9D. In anotherembodiment, the transmitter transmits the pilot from the physicalantennas, without applying the orthonormal matrix U or the permutationmatrix P(k). For this embodiment, a receiver may estimate the actualchannel response based on the pilot and may then derive an effectivechannel response estimate based on the actual channel response estimateand the orthonormal and permutation matrices.

FIG. 10 shows a process 1000 for transmitting data and pilot to one ormore receivers. The processing for each receiver may be performed asfollows. The set of subcarriers assigned to the receiver and the spatialmultiplexing order (M) for the receiver are determined, where M≧1 (block1012). For each assigned subcarrier, M virtual antennas are selected foruse from among V virtual antennas formed with V columns of theorthonormal matrix U, where V≧M (block 1014). The M virtual antennas foreach assigned subcarrier may be selected in various manners, asdescribed above. The output symbols for the receiver are mapped to the Mvirtual antennas selected for each assigned subcarrier by applying theorthonormal matrix (block 1016). The mapped output symbols (or transmitsymbols) are provided for transmission from T transmit antennas, whereT≧V (block 1018).

Pilot symbols are also mapped to the virtual antennas used fortransmission (block 1020). For example, pilot symbols for a common pilotmay be mapped to the first virtual antenna on a first set of pilotsubcarriers, and pilot symbols for an auxiliary pilot may be mapped tothe remaining virtual antennas on a second set of pilot subcarriers.

If there are multiple receivers, then the same or different spatialmultiplexing orders may be used for these receivers. Furthermore, datamay be sent simultaneously on different subcarrier sets to multiplereceivers. For example, data may be sent from one virtual antenna on afirst subcarrier set to a SIMO or SISO receiver, from multiple virtualantennas on a second subcarrier set to a MEMO receiver, and so on. Inany case, the transmit symbols for all receivers are demultiplexed tothe T transmit antennas (block 1022). For each transmit antenna, thetransmit symbols for each receiver are mapped to the subcarriersassigned to that receiver (also block 1022). Transmission symbols arethen generated for each transmit antenna based on the transmit symbolsfor that transmit antenna and using, e.g., OFDM or SC-FDMA (block 1024).Different cyclic delays may be applied for the T transmit antennas,e.g., by circularly delaying the transmission symbols for each transmitantenna by a different amount (block 1026).

For block 1016 in FIG. 10, the output symbol(s) for each subcarrierassigned to each receiver are mapped to the T transmit antennas based onM mapping patterns selected from among V mapping patterns available foruse. Each mapping pattern indicates a specific mapping of an outputsymbol to the T transmit antennas. The V mapping patterns may be formedby V columns of an orthonormal matrix or in other manners. Differentmapping patterns may be selected for different subcarriers in a givensymbol period and/or different symbol periods, e.g., based on apredetermined pattern. The predetermined pattern may be defined by apermutation matrix or in some other manner. The predetermined patternmay cycle through the V available mapping patterns in differentsubcarriers and/or symbol periods.

FIG. 11 shows an embodiment of an apparatus 1100 for transmitting dataand pilot to one or more receivers. Apparatus 1100 includes means fordetermining the set of subcarriers assigned to each receiver and thespatial multiplexing order (M) for each receiver (block 1112), means forselecting M virtual antennas for use from among V virtual antennas foreach subcarrier assigned to each receiver (block 1114), means formapping the output symbols for each receiver to the virtual antennasselected for each subcarrier assigned to the receiver (e.g., by applyingselected columns of an orthonormal matrix or selected mapping patterns)(block 1116), means for providing the mapped output symbols (or transmitsymbols) for transmission from T transmit antennas (block 1118), meansfor mapping pilot symbols to the virtual antennas used for transmission(block 1120), means for demultiplexing the transmit symbols for eachreceiver to the assigned subcarriers of the T transmit antennas (block1122), means for generating transmission symbols for each transmitantenna, e.g., using OFDM or SC-FDMA (block 1124), and means forapplying different cyclic delays for the T transmit antennas (block1126).

FIG. 12 shows a block diagram of an embodiment of base station 110,single-antenna terminal 120 x, and multi-antenna terminal 120 y. At basestation 110, a transmit (TX) data processor 1210 receives data for oneor more terminals, processes (e.g., encodes, interleaves, and symbolmaps) the data based on one or more coding and modulation schemes, andprovides modulation symbols. TX data processor 1210 typically processesthe data for each terminal separately based on a coding and modulationscheme selected for that terminal. If system 100 utilizes SC-FDMA, thenTX data processor 1210 may perform FFT/DFT on the modulation symbols foreach terminal to obtain frequency-domain symbols for that terminal. TXdata processor 1210 obtains output symbols for each terminal (which maybe modulation symbols for OFDM or frequency-domain symbols for SC-FDMA)and multiplexes the output symbols for the terminal onto the subcarriersand virtual antennas used for that terminal. TX data processor 1210further multiplexes pilot symbols onto the subcarriers and virtualantennas used for pilot transmission.

A TX spatial processor 1220 receives the multiplexed output symbols andpilot symbols, performs spatial processing for each subcarrier, e.g., asshown in equation (3) or (7), and provides transmit symbols for the Ttransmit antennas. A modulator (Mod) 1222 processes the transmit symbolsfor each transmit antenna, e.g., for OFDM, SC-FDMA, or some othermodulation technique, and generates an output sample stream for thattransmit antenna. Since TX spatial processor 1220 performs spatialprocessing for each subcarrier, the SC-FDMA modulation is divided intotwo parts that are performed by TX data processor 1210 and modulator1222. Modulator 1222 provides T output sample streams to T transmitterunits (TMTR) 1224 a through 1224 t. Each transmitter unit 1224 processes(e.g., converts to analog, amplifies, filters, and frequency upconverts)its output sample stream and generates a modulated signal. T modulatedsignals from transmitter units 1224 a through 1224 t are transmittedfrom T antennas 112 a through 112 t, respectively.

At each terminal 120, one or multiple antennas 122 receive the modulatedsignals transmitted by base station 110, and each antenna provides areceived signal to a respective receiver unit (RCVR) 1254. Each receiverunit 1254 processes (e.g., amplifies, filters, frequency downconverts,and digitalizes) its receive signal and provides received samples to ademodulator (Demod) 1256. Demodulator 1256 processes the receivedsamples for each receive antenna 122 (e.g., based on OFDM, SC-FDMA, orsome other modulation technique), obtains frequency-domain receivedsymbols for the K total subcarriers, provides received symbols for theassigned subcarriers, and provides received pilot symbols for thesubcarriers used for pilot transmission.

For single-antenna terminal 120 x, a data detector 1260 x obtainsreceived symbols from demodulator 1256 x, derives channel estimates forthe assigned subcarriers based on the received pilot symbols, andperforms data detection (e.g., equalization) on the received symbolsbased on the channel estimates to obtain detected symbols, which areestimates of the output symbols transmitted to terminal 120 x. Formulti-antenna terminal 120 y, a receive (RX) spatial processor 1260 yobtains received symbols from demodulator 1256 y, derives channelestimates for the assigned subcarriers based on the received pilotsymbols, and performs receiver spatial processing on the receivedsymbols based on the channel estimates to obtain detected symbols. RXspatial processor 1260 y may implement a minimum mean square error(MMSE) technique, a zero-forcing (ZF) technique, a maximal ratiocombining (MRC) technique, a successive interference cancellationtechnique, or some other receiver processing technique. For eachterminal, an RX data processor 1262 processes (e.g., symbol demaps,deinterleaves, and decodes) the detected symbols and provides decodeddata for the terminal. In general, the processing by each terminal 120is complementary to the processing by base station 110.

Each terminal 120 may generate feedback information for the datatransmission to that terminal. For example, each terminal 120 mayestimate the SNRs for the virtual antennas, e.g., based on the receivedpilot symbols. Each terminal 120 may select one or more coding andmodulation schemes, one or more packet formats, one or more virtualantennas to use for data transmission, one or more orthonormal matrices,and so on based on the SNR estimates and/or other information. Eachterminal 120 may also generate acknowledgments (ACKs) for correctlyreceived data packets. The feedback information may include the SNRestimates, the selected coding and modulation schemes, the selectedvirtual antenna(s), the selected orthonormal matrix(ces), the selectedsubcarrier(s), ACKs, information used for power control, some otherinformation, or any combination thereof. The feedback information isprocessed by a TX data processor 1280, further processed by a TX spatialprocessor 1282 if multiple antennas are present, modulated by amodulator 1284, conditioned by transmitter unit(s) 1254, and transmittedvia antenna(s) 122 to base station 110. At base station 110, themodulated signals transmitted by terminals 120 x and 120 y are receivedby antennas 112, conditioned by receiver units 1224, and processed by ademodulator 1240, an RX spatial processor 1242, and an RX data processor1244 to recover the feedback information sent by the terminals. Acontroller/processor 1230 uses the feedback information to determine thedata rates and coding and modulation schemes to use for the datatransmission to each terminal as well as to generate various controlsfor TX data processor 1210 and TX spatial processor 1220.

Controllers/processors 1230, 1270 x and 1270 y control the operation ofvarious processing units at base station 110 and terminals 120 x and 120y, respectively. Memory units 1232, 1272 x and 1272 y store data andprogram codes used by base station 110 and terminals 120 x and 120 y,respectively. Controller/processor 1230 may implement parts of FIGS. 10and 11 and may (1) assign subcarriers and select the spatialmultiplexing order for each terminal (block 1012 in FIG. 10) and (2)select the virtual antennas for each subcarrier assigned to eachterminal (block 1214 in FIG. 10). TX data processor 1220 may implementparts of FIGS. 10 and 11 and perform the processing shown in blocks 1116through 1126 in FIG. 10.

For clarity, much of the description above is for a system with K totalsubcarriers. The transmission techniques described herein may also beused for a system with a single subcarrier. For such a system, k in thedescription above may be an index for symbol period instead ofsubcarrier.

The transmission techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, firmware, software, or a combination thereof. For a hardwareimplementation, the processing units at a transmitter may be implementedwithin one or more application specific integrated circuits (ASICs),digital signal processors (DSPs), digital signal processing devices(DSPDs), programmable logic devices (PLDs), field programmable gatearrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof. Theprocessing units at a receiver may also be implemented within one ormore ASICs, DSPs, processors, and so on.

For a software implementation, the transmission techniques may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory (e.g., memory 1230, 1272 x or 1272 y in FIG. 12) andexecuted by a processor (e.g., processor 1232, 1270 x or 1270 y). Thememory may be implemented within the processor or external to theprocessor.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An apparatus, comprising: at least one processor configured toreceive a data vector; select a plurality of virtual antennas to use fortransmission from among a plurality of virtual antennas; form apermutation matrix indicative of the plurality of virtual antennasselected; apply the permutation matrix to the data vector to createoutput symbols; apply different cyclic delays for each respectivetransmit antenna in a plurality of transmit antennas to the outputsymbols to create a transmit vector; provide the transmit vector to theplurality of transmit antennas; and a memory coupled to the at least oneprocessor.
 2. The apparatus of claim 1, wherein the cyclic delays areapplied by circularly shifting a sequence of time-domain samples foreach antenna in the plurality of transmit antennas.
 3. The apparatus ofclaim 1, wherein the cyclic delays are applied by applying a phase rampacross subcarriers for each of the transmit antennas in the plurality oftransmit antennas.
 4. The apparatus of claim 1, wherein the processor isfurther configured to append a cyclic prefix to the transmit vectorafter applying the different cyclic delays.
 5. The apparatus of claim 1,further comprising at least one of a base station and a terminal, withwhich the apparatus of claim 1 is integrated.
 6. A method, comprising:receiving a data vector; selecting a plurality of virtual antennas touse for transmission from among a plurality of virtual antennas; forminga permutation matrix indicative of the plurality of virtual antennasselected; applying the permutation matrix to the data vector to createoutput symbols; applying different cyclic delays for each respectivetransmit antenna in a plurality of transmit antennas to the outputsymbols to create a transmit vector; and providing the transmit vectorto the plurality of transmit antennas.
 7. The method of claim 6, furthercomprising applying the cyclic delays by circularly shifting a sequenceof time-domain samples for each antenna in the plurality of transmitantennas.
 8. The method of claim 6, further comprising applying thecyclic delays by applying a phase ramp across subcarriers for each ofthe transmit antennas in the plurality of transmit antennas.
 9. Themethod of claim 6, further comprising appending a cyclic prefix to thetransmit vector after applying the different cyclic delays.
 10. Anapparatus, comprising: means for receiving a data vector; means forselecting a plurality of virtual antennas to use for transmission fromamong a plurality of virtual antennas; means for forming a permutationmatrix indicative of the plurality of virtual antennas selected; meansfor applying the permutation matrix to the data vector to create outputsymbols; means for applying different cyclic delays for each respectivetransmit antenna in a plurality of transmit antennas to the outputsymbols to create a transmit vector; and means for providing thetransmit vector to the plurality of transmit antennas.
 11. The apparatusof claim 10, wherein the cyclic delays are applied by circularlyshifting a sequence of time-domain samples for each antenna in theplurality of transmit antennas.
 12. The apparatus of claim 10, whereinthe cyclic delays are applied by applying a phase ramp acrosssubcarriers for each of the transmit antennas in the plurality oftransmit antennas.
 13. The apparatus of claim 10, further comprisingmeans for appending a cyclic prefix to the transmit vector afterapplying the different cyclic delays.
 14. The apparatus of claim 10,further comprising at least one of a base station and a terminal, withwhich the apparatus of claim 1 is integrated.
 15. A non-transitorycomputer-readable medium, comprising instructions stored thereon that,if executed by a processor, cause the processor to execute a methodcomprising: receiving a data vector; selecting a plurality of virtualantennas to use for transmission from among a plurality of virtualantennas; forming a permutation matrix indicative of the plurality ofvirtual antennas selected; applying the permutation matrix to the datavector to create output symbols; applying different cyclic delays foreach respective transmit antenna in a plurality of transmit antennas tothe output symbols to create a transmit vector; and providing thetransmit vector to the plurality of transmit antennas.
 16. Thenon-transitory computer-readable medium of claim 15, wherein the methodfurther comprises applying the cyclic delays by circularly shifting asequence of time-domain samples for each antenna in the plurality oftransmit antennas.
 17. The non-transitory computer-readable medium ofclaim 15, wherein the method further comprises applying the cyclicdelays by applying a phase ramp across subcarriers for each of thetransmit antennas in the plurality of transmit antennas.
 18. Thenon-transitory computer-readable medium of claim 15, wherein the methodfurther comprises appending a cyclic prefix to the transmit vector afterapplying the different cyclic delays.
 19. The non-transitorycomputer-readable medium of claim 15, further comprising at least one ofa base station and a terminal, with which the computer-readable mediumof claim 15 is integrated.